Methods for push distraction and for provision of therapy to adjacent motion segments

ABSTRACT

Disclosed are methods and apparatus for the provision of spinal therapy to two or more adjacent motion segments accessed through a trans-sacral approach. The spinal therapies include fusion and dynamic stabilization with and without a distraction of the most cephalad motion segment of the two or more adjacent motion segments provided therapy. The disclosure includes methods and apparatus to impart a push distraction to increase the distance between two adjacent vertebrae. Related concepts for the extraction of previously inserted devices and the delivery and removal of plugs for plugging the interior cavities of implantable devices are disclosed.

This application claims priority and incorporates by reference aco-pending and commonly assigned U.S. patent application Ser. No.11/202,655 filed Aug. 13, 2005 for Methods and Apparatus for Provisionof Therapy to Adjacent Motion Segments. Through the '655 application,this application claims priority and incorporates by reference U.S.Provisional Application No. 60/601,842 filed Aug. 14, 2004 for Method &Apparatus for Multi-Level Stabilization of the Spine and U.S. patentapplication Ser. No. 11/189,943, now U.S. Pat. No. 7,608,077 for MethodAnd Apparatus for Spinal Distraction and Fusion.

This application claims priority and incorporated by referenceco-pending and commonly assigned U.S. patent application Ser. No.12/339,932 for Dual Anchor Prosthetic Nucleus Apparatus which is acontinuation of U.S. patent application Ser. No. 10/972,039 now U.S.Pat. No. 7,491,236 for Dual Anchor Prosthetic Nucleus Apparatus. U.S.patent application Ser. No. 10/972,039 filed Oct. 22, 2004 claimspriority and benefits from co-pending and commonly assigned U.S.Provisional Patent Application No. 60/513,899 filed Oct. 23, 2004 forSurgical Instrumentation and Implants for Spinal Procedures. The presentapplication incorporates by reference application Ser. Nos. 10/972,039and 60/513,899.

This application extends the work done by TranS1 Inc. and incorporatesby reference a set of Unites States applications, provisionalapplications, and issued patents including: Ser. No. 11/189,943 filedJul. 26, 2005 (now issued as U.S. Pat. No. 7,608,077); Ser. No.10/309,416 filed Dec. 3, 2002 (now issued as U.S. Pat. No. 6,921,403);60/182,748 filed Feb. 16, 2000; Ser. No. 09/640,222 filed Aug. 16, 2000(now issued as U.S. Pat. No. 6,575,979); Ser. No. 10/459,149 filed Jun.11, 2003 (now issued as U.S. Pat. No. 7,087,058); Ser. No. 09/684,820filed Oct. 10, 2000 (now issued as U.S. Pat. No. 6,558,386); Ser. No.10/430,751 filed May 6, 2003; 60/182,748 filed Feb. 16, 2000; Ser. No.09/782,583 filed Feb. 13, 2001 (issued as U.S. Pat. No. 6,558,390); Ser.No. 09/848,556 filed May 3, 2001 (now issued as U.S. Pat. No.7,014,633); Ser. No. 10/125,771 filed Apr. 18, 2002 (issued as U.S. Pat.No. 6,899,716); Ser. No. 10/990,705 filed Nov. 17, 2004 (now issued asU.S. Pat. No. 7,329,259); Ser. No. 10/430,841 filed May 6, 2003 (nowissued as U.S. Pat. No. 7,309,338); Ser. No. 09/710,369 filed Nov. 10,2000 (now issued as U.S. Pat. No. 6,740,090); Ser. No. 10/853,476 filedMay 25, 2004 (now issued as U.S. Pat. No. 7,569,056); Ser. No.09/709,105 filed Nov. 10, 2000 (now issued as U.S. Pat. No. 6,790,210);Ser. No. 09/782,534 filed Feb. 13, 2001; 60/513,899 filed Oct. 23, 2003;a series of applications filed Oct. 22, 2004: Ser. No. 10/971,779 (nowissued as U.S. Pat. No. 7,530,993), Ser. Nos. 10/971,781, 10/971,731,10/972,077 (now issued as U.S. Pat. No. 7,500,977), Ser. No. 10/971,765(now issued as U.S. Pat. No. 7,473,256), Ser. Nos. 10/972,065,10/971,775, 10/971,780 (now issued as U.S. Pat. No. 7,588,574), Ser. No.10/972,184 (now issued as U.S. Pat. No. 7,717,958), Ser. No. 10,972,039(now issued as U.S. Pat. No. 7,491,236), Ser. No. 10,972,040 (now U.S.Pat. No. 7,662,173), and Ser. No. 10/972,176 (now issued as U.S. Pat.No. 7,547,324); 60/558,069 filed Mar. 31, 2004; and 60/706,704 filedAug. 9, 2005.

FIELD OF THE DISCLOSURE

The present disclosure relates generally to implantable deviceassemblies, instrumentation systems, and methods for accessing andachieving axial stabilization at multiple levels of the spine via aminimally-invasive trans-sacral approach (as described in U.S. Pat. No.6,558,390 which is incorporated herein by reference) and subsequenttherapeutic procedures, such as spinal arthroplasty; partial or totaldisc replacement; annulus repair, vertebroplasty; arthrodesis (fusion),or other spine-related procedures comprising the deployment of distaland proximal elongated implantable components and assemblies that can beused to position, manage motion, and stabilize a plurality of adjacentvertebral motion segments in the human spine to relieve lower back pain,restore physiological function of the lumbar spine, to maintain andpossibly improve disc health, and prevent progression or transition ofdegenerative disease. More specifically, the present disclosuregenerally relates to the imposition of a sequence of two or moredistractions on a set of two or more adjacent motion segments as part ofthe provision of therapy to the spine. Alternatively, the presentdisclosure includes methods of distracting a second, more caudalintervertebral disc space after placement of a therapeutic device fordistraction and/or therapy of, or in, an adjacent distal motion segment.While the concept of distraction can be applied for moving one itemapart from another in any dimension, in the context of this applicationand the claims that follow, distraction is considered in the orientationof the axes of the spinal column so that distraction elevates the heightof, i.e., increases the distance between two adjacent vertebral bodiesas measured in the direction of the longitudinal axis of the spine.

BACKGROUND Overview

The present disclosure is an extension of work assigned to TranS1 Inc.with a principle place of business located in Wilmington, N.C. Much ofthe work is described in great detail in the many applicationsreferenced above and incorporated by reference into this application.Accordingly, the background of the disclosure provided herein does notrepeat all of the detail provided in the earlier applications, butinstead highlights how the present disclosure adds to this body of work.

The spinal column is a complex system of bone segments (vertebral bodiesand other bone segments) which are in most cases separated from oneanother by discs in the intervertebral spaces. FIG. 1 shows the varioussegments of a human spinal column as viewed from the side. Each pair ofadjacent vertebral bodies and the intervertebral space contributes tothe overall flexibility of the spine (known as a motion segment) andcontributes to the overall ability of the spine to flex to providesupport for the movement of the trunk and head. The vertebrae of thespinal cord are conventionally subdivided into several sections. Movingfrom the head to the tailbone, the sections are cervical 104, thoracic108, lumbar 112, sacral 116, and coccygeal 120. The individual segmentswithin the sections are identified by number starting at the vertebralbody closest to the head. Of particular interest in this application arethe vertebral bodies in the lumbar section and the sacral section. Asthe various vertebral bodies in the sacral section are usually fusedtogether in adults, it is sufficient and perhaps more descriptive tomerely refer to the sacrum rather than the individual sacral components.

The individual motion segments within the spinal columns allow movementwithin constrained limits and provide protection for the spinal cord.The discs are important to allow the spinal column to be flexible and tobear the large forces that pass through the spinal column as a personwalks, bends, lifts, or otherwise moves. Unfortunately, for a number ofreasons referenced below, for some people one or more discs in thespinal column will not operate as intended. The reasons for discproblems range from a congenital defect, disease, injury, ordegeneration attributable to aging. Often when the discs are notoperating properly, the gap between adjacent vertebral bodies is reducedand this causes additional problems including pain.

A range of therapies have been developed to alleviate the painassociated with disc problems. One class of solutions is to remove thefailed disc and then fuse the two adjacent vertebral bodies togetherwith a permanent but inflexible spacing, also referred to as staticstabilization. Fusing one section together ends the ability to flex inthat motion segment. However, as each motion segment only contributes asmall portion of the overall flexibility of the spine, it can be areasonable trade-off to give up the flexibility of a motion segment inan effort to alleviate significant back pain.

Another class of therapies attempts to repair the disc so that itresumes operation with the intended intervertebral spacing andmechanical properties. One type of repair is the replacement of theoriginal damaged disc with a prosthetic disc. This type of therapy iscalled by different names such as dynamic stabilization or spinalmobility preservation.

Generally, one of the first steps in providing either type of therapy(fusion or motion preservation) is to move adjacent vertebral bodiesrelative to one another (called distraction) to compensate for thereduction of intervertebral space attributed to the problems with thedisc. Depending on the type of therapy that is to be delivered, it maybe useful to separate the adjacent vertebral bodies by more than anormal amount of separation.

It is useful to set forth some of the standard medical vocabulary beforegetting into a more detailed discussion of the background of the presentdisclosure. In the context of the this discussion: anterior refers to infront of the spinal column; (ventral) and posterior refers to behind thecolumn (dorsal); cephalad means towards the patient's head (sometimes“superior”); caudal (sometimes “inferior”) refers to the direction orlocation that is closer to the feet. As the present applicationcontemplates accessing the various vertebral bodies and intervertebralspaces through a preferred approach that comes in from the sacrum andmoves towards the head, proximal and distal are defined in context ofthis channel of approach. Consequently, proximal is closer to thebeginning of the channel and thus towards the feet or the surgeon,distal is further from the beginning of the channel and thus towards thehead, or more distant from the surgeon.

After the preceding primer on the subject, it is thought appropriate anduseful to provide a more detailed discussion of the background of thedisclosure.

Detailed Background of the Disclosure.

Chronic lower back pain is a primary cause of lost work days in theUnited States, and as such is a significant factor affecting bothworkforce productivity and health care expense. There are currently over700,000 surgical procedures performed annually to treat lower back painin the U.S. In 2004, it is conservatively estimated that there will bemore than 200,000 lumbar fusions performed in the U.S., and more than300,000 worldwide, representing approximately a $1B endeavor in anattempt to alleviate patients' pain. About 80% of the procedures involvethe lower lumbar vertebrae designated as the fourth lumbar vertebra(“L4”), the fifth lumbar vertebra (“L5”), and the sacrum. In addition,statistics show that only about 70% of these procedures performed willbe successful in achieving pain relief. Persistent lower back pain isgenerally “discogenic” in origin, i.e., attributed primarily toherniation and/or degeneration of the disc located between the L5-sacrumand/or the L4-L5 vertebral bodies in the lower lumbar section of thespine (See element 112 in FIG. 1).

Degeneration of the disc occurs when the intervertebral disc of thespine suffers reduced mechanical functionality due to dehydration of thenucleus pulposus. The nucleus pulposus provides for cushioning anddampening of compressive forces to the spinal column. In a healthy adultspine, the nucleus pulposus comprises 80% water. With age, the water andprotein content of this tissue and the body's cartilage changesresulting in thinner, more fragile cartilage. Hence, the spinal discsand the facet joints that stack the vertebrae, both of which are partlycomposed of cartilage, are subject to similar degradation over time. Thegradual deterioration of the disc between the vertebrae is known asdegenerative disc disease, or spondylosis. Spondylosis is depicted onx-ray tests or MRI scanning of the spine as a narrowing (heightreduction) of the normal “disc space” between adjacent vertebrae.

The pain from degenerative disc or joint disease of the spine may betreated conservatively with intermittent heat, rest, rehabilitativeexercises, and medications to relieve pain, muscle spasm, andinflammation, but if these treatments are unsuccessful, progressivelymore active interventions may be indicated. In the context of thepresent disclosure, therapeutic procedures to alleviate pain and restorefunction are described in a progression-of-treatment from spinalarthroplasties, comprising prosthetic nucleus device implantation;annulus repair, and total disc replacement, to spinal arthrodesis, i.e.,fusion, with or without concomitant device implantation.

Fusion involves a discectomy, i.e., surgical removal of the disc,followed by the subsequent immobilization of the two vertebral bodies,one superior and one inferior to the excised disc. Collectively, thisunit of two vertebral bodies separated axially by a spinal disc,comprise a spinal motion segment. This procedure of discectomy and“fusion” of the vertebral bodies, i.e., so that the two vertebraeeffectively become one solid bone, terminates all motion at that jointand is intended to eliminate or at least ameliorate discogenic pain. Thebenefit of fusion is pain relief and the down side is elimination ofmotion at the fused joint, which can hinder function. This surgicaloption is generally reserved for patients with advanced discdegeneration, and surgical procedures, such as spinal fusion anddiscectomy, may alleviate pain, but do not restore normal physiologicaldisc function.

Moreover, traditional surgical fusion and other procedures that involvethe removal of the herniated disc, e.g., with laminotomy (a small holein the bone of the spine surrounding the spinal cord); laminectomy(removal of the bony wall); percutaneous discectomy (needle techniquethrough the skin), or chemonucleolysis (disc-dissolving) generally haveaccessed the lumbo-sacral spine via direct, open exposure of theanterior or posterior segments, which limit and dictate the nature anddesign of instrumentation and implants used for site access andpreparation; disc decompression (e.g., distract or elevate vertebralbodies in a motion segment to restore intervening disc height);fixation, and bone growth materials augmentation to facilitate thefusion process. Typical anterior or posterior surgical approaches anddevice interventions generally involve lateral screw fixation to thevertebral bodies of the lumbar spine and sacrum and various types offasteners (rods, plates, etc.) that connect these screws together as onelarge construct. Such approaches also require muscle and ligamentdissection, neural retraction, and annular disruption, i.e., are highlytissue invasive.

More recent anterior fixation systems have reduced the profile of thesedevices by locating their connector rods inside the anterior vertebralcolumn instead of on the outside. In this manner, these systems mayreduce or eliminate exposed surfaces for impingement of nerves, vessels,or soft tissue following implantation.

As an alternative therapy to spinal fusion, i.e., immobilization of thevertebral bodies within a motion segment, axial spinal mobilitypreservation devices (or herein, also termed motion management, or “MM”devices) generally introduced percutaneously through tissue to atrans-sacral access point on the spine in a minimally invasive, lowtrauma manner, to provide therapy to the spine, were disclosed inco-pending and commonly assigned U.S. Provisional Patent Application No.60/558,069 filed Mar. 31, 2004, incorporated herein in its entirety intothis disclosure by reference. The therapeutic advantage of the MMdevices is to preserve mobility by restoring and managing motion via insitu dynamic device performance, and in a preferred aspect, the MMdevices are assemblies that comprise a prosthetic nucleus (PN) componentconfigured as an expandable membrane which is filled in situ, e.g., byinjection or infusion of prosthetic nucleus material (PNM) withviscoelastic properties that assist in distraction (i.e., restoring discheight), and share and distribute physiologic loads among thephysiologic structures of the vertebral motion segment, includingdistribution to the annulus and the inferior and superior vertebral boneend plates, so that biomechanical properties and function are optimized.

In order to overcome shortcomings or limitations associated with priorspinal devices and therapies, the present disclosure discloses novelaxial spinal stabilization systems, comprising device assemblies anddeployment instrumentation, that facilitate treatment among a pluralityof vertebrae and motion segments at multiple levels of the spine, eitherby means of fixation or motion management constructs or via acombination of these therapies, which assemblies are introduced via aminimally invasive, pre-sacral access tract and trans-sacral axialsurgical approach to the lumbo-sacral spine. More specifically, theaxial rods and device assemblies are implanted preferably into theanterior vertebral column by means of device deployment instrumentationto systematically achieve novel means for multi-level axial spinalstabilization via therapeutic intervention or combination ofinterventions as indicated which selectively target multiple, adjacentmotion segments to immobilize vertebral bodes and/or dynamically restoreincreased range of motion, improve biomechanical function, and providediscogenic pain relief.

In the context of the present disclosure, a “motion segment” comprisesadjacent vertebrae, i.e., an inferior and a superior vertebral body, andthe intervertebral disc space separating said two vertebral bodies,whether enucleated space or with intact or damaged spinal discs.

Axial Trans-Sacral Access.

Axial trans-sacral access to the lumbo-sacral spine as shown in FIG. 2,eliminates the need for muscular dissection and other invasive stepsassociated with traditional spinal surgery while allowing for the designand deployment of new and improved instruments and therapeuticinterventions, including stabilization, mobility preservation, andfixation devices/fusion systems across a progression-of-treatment inintervention. More specifically, clinical indications for themulti-level axial spinal stabilization systems and the motion managementassemblies described herein include patients requiring interventions totreat pseudoarthrosis, revisions of previous interventions, spinalstenosis, spondylolisthesis (Grade 1 or 2), or degenerative disc diseaseas defined as back pain of discogenic origin with degeneration of thedisc confirmed by history and radiographic studies. The nature ofspecific assemblies selection by clinicians is dictated by multiplefactors, e.g., individual patient age, anatomy, and needs within theprogression-of-treatment options available, as well as in accordancewith optimal intended function and in deference to biomechanical andsafety constraints. FIG. 2 provides an introductory overview of theprocess with FIGS. 2A and 2B showing the process of “walking” a blunttip stylet 204 up the anterior face of the sacrum 116 to the desiredposition on the sacrum 116 while monitored on a fluoroscope (not shown).This process moves the rectum 208 out of the way so that a straight pathis established for the subsequent steps. FIG. 2C illustrates arepresentative axial trans-sacral channel 212 established through thesacrum 116, the L5/sacrum intervertebral space, the L5 vertebra 216, theL4/L5 intervertebral space, and into the L4 vertebra 220.

The use of a trans-sacral approach to provide spinal therapy isdescribed in co-pending and commonly assigned U.S. patent applicationSer. No. 10/309,416 which is incorporated by reference into thisapplication. A brief overview of this method of accessing the spinalregion to receive therapy is useful to provide context for the presentdisclosure. As shown in FIG. 2A, a pre-sacral approach throughpercutaneous anterior track towards sacral target, through whichtrans-sacral axial bore will be made and the access channel extendeddistally for subsequent advancement of multi-level axial spinalstabilization assemblies. An anterior, pre-sacral, percutaneous tractextends through the pre-sacral space anterior to the sacrum. Thepre-sacral, percutaneous tract is preferably used to introduceinstrumentation to access and prepare the access channel (e.g., bydrilling a bore in the distal/cephalad direction through one or morelumbar vertebral bodies and intervening discs). “Percutaneous” in thiscontext simply means through the skin and to the posterior or anteriortarget point, as in transcutaneous or transdermal, without implying anyparticular procedure from other medical arts. However, percutaneous isdistinct from a surgical access, and the percutaneous opening in theskin is preferably minimized so that it is less than 4 cm across,preferably less than 2 cm, and, in certain applications, less than 1 cmacross. The percutaneous pathway is generally axially aligned with thebore extending from the respective anterior or posterior target pointthrough at least one sacral vertebral body and one or more lumbarvertebral body in the cephalad direction as visualized by radiographicor fluoroscopic equipment.

More specifically, as shown in FIG. 2B, the lumbar spine is accessed viaa small skin puncture adjacent to the tip of the coccyx bone. Thepre-sacral space is entered, using standard percutaneous technique, andthe introducer assembly with the stylet's blunt tip serving as a dilatoris placed through the paracoccygeal entry site. Once the tip of thestylet is through the facial layer, the blunt tip is rotated backagainst the anterior face of the sacrum and “walked” to the desiredposition on the sacrum under fluoroscopic guidance. Once the target sitehas been accessed and risk of soft tissue damage mitigated, theblunt-tipped stylet is removed and a guide pin, or wire, is safelyintroduced through the guide pin introducer tube, and “tapped in”. Theguide pin establishes the trajectory for placement of subsequent bonedilators and sheath through which a twist drill is introduced creatingan axial bore track, the lumen of which is extended distally. The guidepin maintains the axial alignment of access and preparation tools aswell as the alignment of cannulated spinal stabilization devices andassemblies, of larger diameter than the bore track, that aresubsequently introduced over a 23″ long, 0.090″ diameter guide pin andthrough an exchange cannula for deployment within the vertebral column,as described at least in part in co-pending and commonly assigned U.S.patent application Ser. Nos. 10/972,065, 10/971,779, 10/971,781,10/971,731, 10/972,077, 10/971,765, 10/971,775, 10/972,299, and10/971,780, all of which were filed on Oct. 22, 2004, and in co-pendingand commonly assigned United States Provisional Patent Application“Method & Apparatus for Access & Deployment of Spinal StabilizationDevices Through Tissue”, attorney docket no. TranS1 050809 LLG-8, filedAug. 9, 2005, and all of which are incorporated by reference herein intheir entirety.

As used herein, spinal arthrodesis is used in the context offixation/fusion, leading to immobilization of two vertebral bodieswithin a motion segment, relative to one another. In the context of thepresent disclosure, “soft fusion” refers to immobilization by means ofthe introduction of bone growth facilitation/augmentation materials(e.g., osteogenic; osteoconductive media) without accompanyingimplantation of fixation devices (e.g., fusion rods).

In contrast, as used herein, the terms spinal arthroplasty and/or motionmanagement encompass dynamic stabilization options for treating discdegeneration in a progression-of-treatment, i.e., when fusion is deemedtoo radical an intervention based on an assessment of the patient's age,degree of disc degeneration, and prognosis. More specifically, in thecontext of the present disclosure dynamic refers to non-static motionmanagement devices inherently configured to allow mobility by enablingor facilitating forces or load bearing that assist or substitute forphysiological structures that are otherwise compromised, weakened orabsent. Mobility devices providing dynamic stabilization (DS) areprovided across a progression-of-treatment for treating symptomaticdiscogenic pain, ranging from treatment in patients where littledegeneration or collapse is evident radio-graphically, to those for whomprosthetic nucleus devices or total disc replacements are indicated. Forexample, a prosthetic nucleus (PN) would be indicated in patients with agreater degree of degeneration and loss of disc height but not to thestage where advanced annular break-down is present. A PN would go beyondDS by including an aggressive nucleectomy and subsequent filling of thede-nucleated space with an appropriate material. When introducing aprosthetic nucleus (including TDR—total disc replacement or PDR partialdisc replacement), the goal is to restore, as opposed to preserve, discheight and motion. Total disc replacement (TDR) would be indicated withmore advanced disease than with a PN but where some annular functionremains.

In accordance with the present disclosure, therapeutic dynamicstabilization device assemblies are disclosed which provide, by design,resistance and limitation of motion in a controlled manner. As usedherein, “resistance” refers to the force required to move through a fullrange of motion, whereas in contrast, “limitation” refers to not forcebut degree, i.e., curtailment of full range of motion in one or moredirections. With respect to the lower levels of the lumbar spine, fullrange of motion comprises about 12 degrees of flexion, about 8 degreesof extension, about 4 degrees of left or right lateral bend, and about 2degrees of clockwise or counterclockwise motion at each motion segment.Biomechanical properties of the mobility devices may be altered bydesign. For example, a flex coupler embodiment which supports a staticload of about 100 lbs. at 1 mm deflection may be modified, by changing(reducing) the number of coils per turn, moving from a spring constantof about 2500 psi, or by altering “waist” diameter, i.e., the crosssectional area of the flexible mid-section of the device, which causesit to be stiffer, to withstand 200 lbs. at 1 mm deflection.

Thus, devices may be preferentially configured to comprise, for example,a mechanical stop(s) to limit motion, and/or resistance to motion, e.g.,by varying cross-sectional area and hence stiffness, or otherbiomechanical properties relevant to preserving or restoringphysiological function with respect to mobility. The assemblies may beconstructed to provide full, unconstrained range of motion,semi-constrained range of motion where full range of motion is allowedin combination with increased resistance to motion, or limited range ofmotion wherein the extent of motion in one or more degrees of freedom ismechanically limited, with or without increased resistance to motion.More specifically, the spinal devices comprised in the inventiveassemblies preferably selectively approximate the biomechanicalproperties (e.g., substantially matched bulk and compression modulus) ofthe physiological vertebral or disc structure(s) depending on theparticular function(s) for which specific therapeutic procedure(s) areindicated.

In one aspect of the present disclosure, an axial spinal MM devicecomprises PN augmentation or replacement material, that provides thesame load-bearing functions and characteristics as the natural discnucleus and of the natural disc, said PN material contained withinexpandable membranes, comprised of elastomeric materials, e.g.,silicone. Exemplary silicone is such as that obtained from NusilSilicone Technology located in Carpeneria, Calif., exhibiting elongationof between about 500% and about 1500%, and most preferably at about1000%, and having a wall thickness of 0.015″ serve as a primary dynamicstabilization component, via load assimilation and load distribution,when filled and expanded via infusion or inflation with an appropriatematerial. In a further aspect of the present disclosure, the spinal MMdevice may be configured via engagement means as part of an inter-axialdevice assembly comprising a plurality of axial MM devices, or asassemblies comprising some combination of axial MM device(s) and axialfixation rod(s). In a preferred aspect, this is achieved by means ofaxial deployment of devices with an aspect ratio of greater than 1,i.e., the device dimension in the axial vertebral plane is greater thanthe device dimension in any orthogonal direction to that axial plane inclose proximity to the physiological instantaneous center of axialrotation.

As used herein, the term “axial rod” refers to axially deployed spinalimplants which are fabricated, for example, by machining from metal,cylindrical (i.e., rod-like) solid blanks, and said term may encompassfixation/fusion or motion management devices as indicated, since thespecific nature, form and function of such devices are determinedby/dependent upon final implant configuration. The fabrication, forms,and function of various implant configurations of axial spinal motionmanagement devices to preserve or restore mobility are disclosed inco-pending and commonly assigned U.S. patent application Ser. Nos.10/972,184, 10/972,039, 10/972,040, and 10/972,176, all of which werefiled on Oct. 22, 2004 the contents of which are hereby incorporated intheir entirety into this disclosure by reference.

The axial rods serve multiple purposes, including but not limited to,modifying the height between the bodies, assuming physiological axialloads, providing access for the introduction of osteogenic and/orosteoconductive materials, and precluding device expulsion by means ofanchoring. For example, the method of using the distraction/fusion rodgenerally comprises the steps of: determining the desired change in discheight between targeted vertebral bodies; selecting a rod with theappropriate thread pitches in the distal and proximal sections toachieve the desired change in height; accessing the targeted bodies bycreating an axial bore that extends in the distal (cephalad) directionfrom a target point on the anterior surface of the sacrum to the discspace between the targeted bodies; extending the axial bore in thedistal direction to create an extended portion of the axial bore,wherein the extended portion has a smaller diameter than the portion ofthe axial bore extending from the sacral target point to the disc spacebetween the targeted bodies; and advancing and implanting the selectedrod into the targeted bodies to achieve the desired change in discheight. Moreover, when devices and assemblies are anchored in bone toeliminate migration and expulsion they are preferably configured withself-tapping, bone anchoring threads configured to distribute stressevenly over a large surface area. The threads are typically of“cancellous” type bone threads known in the art. More specifically, theyare typically but not exclusively cut with generally flat faces on theflights of the thread with the flattest of the faces oriented in thedirection of the applied load.

There are a number of parameters that can be used to describe a set ofthreads. A set can be male or female A set of threads can be righthanded or left handed. The number of threads per unit length (pitch) canbe varied from one set of threads to another. The minor and majordiameters of the threads can be varied from one set of threads toanother. A more subtle difference is that form of the threads—the shapeof a cross section of a thread can vary from one set of threads toanother (such as V-shaped threads or buttress threads). For the purposesof this application and the claims that follow, one set of threads issaid to be the same type as another set of threads when all of theseparameters are the same such that the another set of threads can berotated into a thread path cut by the first set of threads withoutneeding to cut a new thread path (if the another set of threads is keyedor timed to place the another set of threads into the proper position tostart into the previously cut thread path).

In a preferred aspect of the disclosure, stop flow means such as anaxial rod plug are used to preclude leakage or migration of theprosthetic nucleus material either through an axial spinal dynamicstabilization rod or from the intervertebral disc space. As will becomeapparent from the accompanying figures and as used herein, “assembly”may refer, in context, to a single implant which when fully deployedwithin the spine comprises at least two distinct parts that areconfigured and engaged in and referred to as an intra-axial alignment,for example, an axial rod and an axial rod plug internally engaged andaxially aligned within (i.e., longitudinally) said axial rod.Additionally, again in context, “assembly” may refer to the combinationof a plurality of single-part implants and/or two part intra-axialdevices-assemblies, such as just described above, which are configuredwith engagement means enabling constructs as inter-axial components thatcollectively comprise an integrated unit or assembly, for example, adistal component rod or rod-assembly as the distal implant in engagementalong the center line of a longitudinal axis and axially aligned with aproximal component rod or rod-assembly, as the proximal implant whereinthe two components collectively comprise a two-level axial stabilizationassembly for two adjacent motion segments, e.g., L4-L5 (distal) andL5-sacrum (proximal).

In a preferred aspect of the present disclosure, multi-level axialstabilization assemblies are configured from two components: a distalcomponent rod, comprising a threaded distal end with a first threadpitch and a threaded proximal end with a second, different thread pitch(hereinafter referred to as dissimilar thread pitches); and a proximalcomponent rod, comprising a distal end that is a tapered andnon-threaded cylinder and a threaded proximal end comprising tapereddistal threads; said component rods (with or without accompanyingintra-axially engaged rod plugs) which are sequentially deployed bymeans of instrumentation and methods as will be described below, asfixation implants in adjacent motion segments, e.g., first in L4-L5(superior/distal component rod) and then L5-sacrum (inferior/proximalcomponent rod), respectively; so that the subsequent engagement of thedistal end of the proximal component rod internally within and ininter-axial alignment with the proximal end of the distal component rodforms a two-level, spinal axial stabilization assembly that enablesindependent (adjusted) axial distraction (an extension/increase inheight, resulting in disc decompression and pain relief) of both theproximal and distal intervertebral discs spaces, respectively, withintwo adjacent motion segments.

In particular, the axial configuration of the anchors (i.e.,self-tapping threads) allows the proximal and distal thread profiles ofthe distal component rod to be of different pitch. Thread pitch, as usedherein, is defined as the distance between corresponding points onconsecutive threads, i.e., threads per inch or TPI. This design usingdissimilar thread pitches allows each end of the rod to screw into thesuperior and inferior vertebral bodies of the L4-L5 motion segment atindependent rates resulting in distraction of the two vertebrae and anincrease in disc height without the need for additional “distracting”instrumentation as is required in other arthrodesis. Moreover, thedegree or amount of distraction to be achieved, for example from betweenabout 1 mm to about 10 mm and often between about 2 mm and 6 mm, may beselected by pre-determining the variability in thread pitch between thethreaded distal and proximal ends.

The use of dissimilar thread pitches to distract vertebral bodies withina single motion segment is described in commonly assigned U.S. Pat. No.6,921,403 “Method and Apparatus for Spinal Distraction and Fusion”issued on Jul. 26, 2005 which is incorporated herein in its entirety byreference into this disclosure. However, in a further inventive aspectof the present multi-level stabilization system, in order to enableadequate and simultaneous distraction and subsequent therapy of a secondmotion segment (proximal) disc space, the proximal component rod needsonly to be threaded at its proximal end as described in the precedingparagraph, so that as its threads engage the proximal vertebral bodysaid component is both anchored and advanced into the proximal discspace until its distal end subsequently engages the distal componentrod's proximal end, effectively comprising an integral implant assembly.In this manner, distraction of the proximal disc space is thereafterachieved by means of force applied, in the distal direction, to theproximal end of the distal component subsequent to said engagement ofthe two components, so that the distal end of the proximal componentwill push against and lift the proximal end of the distal component.

In one aspect of the disclosure, the device assemblies are configured tomechanically and adjustably, distract multiple disc spaces andconfigured to be deployed so as to be oriented in approximately the lineof principal compressive stress, i.e., the device is configured to beplaced at approximately the center of rotation in a human disc motionsegment. In turn, this yields a more uniform, radial distribution ofloads to more closely approximate physiological load sharing.

In accordance with this aspect of the present disclosure, the axialstabilization devices disclosed herein are less likely to cause thephenomena of subsidence and transition syndrome. As used herein,subsidence refers to the detrimental descent of an orthopedic implantinto bone that surrounds it. Transition syndrome refers to alteredbiomechanics and kinematics of contiguous vertebral levels andconcomitant risk of adjacent motion segment instability that may occuras a result of spinal therapeutic procedures that are suboptimal interms of their ability to restore physiological function and properties,and thus risk a cascading deleterious effect on surrounding otherwisehealthy tissue.

Applicants believe the advantage of adjusting distraction between andamong successive adjacent vertebral bodies within multiple motionsegments at various spinal levels, as just described, of the inventivemulti-level axial stabilization assembly systems described herein to beunique, i.e., that no other (known) current spinal therapies are able toachieve across a plurality of adjacent motion segmentsdistraction/decompression and stabilization/motion management, includingcombinations of progression-of-treatment options, leading to discogenicpain relief.

Thus, it is one object of the present disclosure to providedevice-assemblies and deploy them in a method that independentlyincreases intervertebral disc height within a first motion segment and asecond, adjacent motion segment.

It is another object of the present disclosure to provide spinal axialstabilization system assemblies as disclosed herein that restore normalintervertebral disc height by distracting vertebral bodies within andamong a plurality of adjacent motion segments, and to achieve mechanicalstability of the joint by augmenting or replacing prosthetic nucleusmaterial to distribute physiologic loads and/or managing motion in saidspinal segments to eliminate chronic pain.

It is another object of the present disclosure to preserve biomechanicalfunction and eliminate chronic pain by facilitating successful fusion ofmotion segments within multiple levels of the spine by means ofaxially-deployed, differentially threaded (anchored) spinal fixationassemblies that provide adjustable distraction to restore normalintervertebral disc height among a plurality of adjacent motion segmentsand that achieve stabilization in closer proximity to the instantaneouscenter of rotation around the vertical axis of the spine, advantages notafforded by other current spinal fusion systems.

It is a further object of the present disclosure to provide axial spinaldevices and assemblies, as well as instrumentation and methods for theirdeployment, which collectively comprise an axial spinal stabilizationsystem, in particular, for the anterior lumbar spine, capable ofdistracting and treating multiple vertebral bodies and adjacent motionsegments at multiple levels of the spine via fixation; motionmanagement; or both static and dynamic stabilization, by means of aminimally invasive, pre-sacral surgical approach and trans-sacraldeployment and axial orientation of the spinal devices and assembliesthrough the vertebral bodies, in a manner that does not compromise theannulus and adjacent tissues. More specifically, yet another advantageof the present disclosure is the concurrent implementation of acombination of therapies, i.e., deployment of spinal assemblies aredisclosed that enable dynamic stabilization via implantation of one ormore prosthetic nucleus devices or other mobilitypreservation/restoration devices, such as those disclosed and describedpreviously in co-pending and commonly assigned U.S. patent applicationSer. Nos. 10/972,184, 10/972,039, 10/972,040, and 10/972,176, all ofwhich were filed on Oct. 22, 2004 incorporated herein in their entiretyinto this disclosure by reference, as alternative options to or togetherwith fixation rods facilitating fusion of the vertebral bodies, toselectively achieve motion management rather than elimination of motionwith respect to a targeted plurality of motion segments within multiplespinal levels.

MM devices (also referred to as mobility devices) decompress the discand alleviate pain caused by nerve impingement, usually posterior, bymeans of either inducing slight segmental kyphosis (introduction ofadded convex curvature through increasing the height on the posteriorside of the disc more than on the anterior side of the disc) or straightelevation, and by creating limits and resistance to segmental motion. Inthis manner, devices are able to provide both stable anterior andposterior load support (e.g., loads that may approximate 10 times thebody weight of a patient) and adequate medial-lateral and rotationalsupport, without adjunctive posterior instrumentation and withoutaccompanying osteogenesis.

Certain of the dynamic stabilization devices of the present disclosurecomprise a flexible member in between more rigid distal and proximalthreaded anchor portions. The flexible member, which may comprise acable, spring, flexible coupler, stacked-washers, inflatable bladder(e.g., expandable membrane), or a combination thereof, serves as a“shock absorber”, and is able to assimilate forces or redistributeloads. Hence, in accordance with this aspect of the present disclosure,the mobility device assembly comprising one or more flexible member(s),in combination with at least one anchor portion(s), may be configuredfrom among these design concepts and embodiments, including: helicalflexure (flexible coupler) designs, comprising one-piece or two-piecedevices that may be configured with or without an integral, elastomericor elastic inflatable, i.e., expandable, membrane that serves tomaximize surface area over which loads are distributed, and that may ormay not assist in distraction; cable designs, comprising one piece offixed length, with or without an inflatable membrane, or two or moreparts of variable length; ball and track multi-part designs; “stackedwasher” designs, and anchored nuclear replacements.

It is another object of the present disclosure to provide a spinal PND(prosthetic nucleus device) which preferably does not impede themobility of, and is responsive to the physiological ICOR (instantaneouscenter of rotation). Moreover, in one aspect, the PND providesanterior-posterior translation and has a mobile ICOR. The PNDs of thepresent disclosure do not adversely impact the stiffness of the motionsegment being treated. For example, PND axially deployed in theL5-sacrum lumbar spine enable/accommodate range of motion of betweenabout 10° to 15° flexion; between about 7° to about 10° extension; about5° of left or right lateral bending and between about 1° to about 2°clockwise or counterclockwise axial rotation, while those implanted inL4-L5 enable/accommodate range of motion of between about 8° to 10°flexion; between about 5° to about 7° extension; between about 5° toabout 7° left or right lateral bending; and between about 1° to about 4°clockwise or counterclockwise axial rotation.

In a preferred aspect of the disclosure, the overall length of, forexample of the proximal component (L5-Sacrum) of the MM device-assemblyranges from about 40 mm (size small) to about 60 mm (size large), andthe expandable membrane component may be folded within a cannulatedsection of the mobility device during device delivery to the targetsite, and then deployed, e.g., unfolded, in situ via expansion byinfusion or inflation into the (denucleated) intervertebral disc spaceof the L5-Sacrum motion segment.

Thus, preferential vertebral body positioning, distraction anddecompression, and static or dynamic stabilization are achieved byinterventions which at the same time mitigate surgical risks associatedwith traditional, conventional procedures, e.g., bleeding, neurologicaldamage, damage to soft tissue, spinal cord impingement or damage andinfection, and, additionally, provide an improved level of clinicalbiomechanical performance compared with conventional spinal componentsand techniques for spinal arthrodesis or arthroplasties on multiplelevels within the spinal column.

It is further believed that in addition to providing devices andassemblies that can mechanically eliminate or limit acute pathologicmotion and establish long-term stability of spinal segments byimmobilizing or significantly managing the range of motion of thesegment, inherent risks associated with implant breakage, loosening orexpulsion of the implants possibly causing delayed nerve rootimpingement or damage, fracture of osseous structures, and bursitis, aresubstantially reduced with respect to the present inventive axialassemblies, as are pain, discomfort or abnormal sensations due to thepresence of the device.

For example, another advantage of the inventive spinal axialstabilization system is that deployment and orientation present noexposed surfaces for impingement of nerves, vessels, or soft tissue.Additionally, due to the axial delivery of the implant via a protectedchannel, there is no retraction of muscles and no exposure to majorvessels or soft tissue as with the delivery system for systems deliveredfrom other surgical approaches.

This is the case whether the method of implant deployment is by ananterior (preferred), or a posterior approach, and it will be understoodthat references to anterior approaches, while preferred, are forconvenience only, and that both pre-sacral anterior and posteriorapproaches and subsequent trans-sacral axial stabilization methods anddevices afford significant advantages over current practice, including:the patient is in a prone position that is easily adaptable to otherposterior instrumentation; blood loss is minimal; soft tissuestructures, e.g., veins, arteries, nerves are preserved, andsubstantially less surgical and anesthesia time is required comparedwith conventional procedures; the implants of the present disclosure areintended to preserve or restore function, not merely alleviate pain.

The objects, advantages and features of the present disclosure presentedabove are merely exemplary of some of the ways the disclosure overcomesdifficulties presented in the prior art, and are not intended to operatein any manner as a limitation on the interpretation of the disclosure.These and other advantages and features of the multi-level axialstabilization devices and assemblies, as well as the surgical tools setsand techniques for their deployment, disclosed in the present disclosurewill be more readily understood from the following summary and adetailed description of the preferred embodiments thereof, whenconsidered in conjunction with the accompanying figures.

SUMMARY OF THE DISCLOSURE

Previous work has developed a range of fusion and mobility maintenance(MM) therapeutic devices for use in the intervertebral space between theL5 and sacrum. In some instances there may be a need to provide therapyto both the L5/sacrum intervertebral space but also to the adjacentsuperior L4/L5 space and to do so in a manner that requires independentcontrol over the amount of distraction applied to each space. In someinstances there may be a need to provide distraction and then therapy tomore than two adjacent intervertebral spaces such as L3/L4, L4/L5, andL5/sacrum. Further, even if it were possible to perforin a sequence ofsingle distractions using axial rods of the type described above forsingle level distraction (as there are challenges to placement of twodifferent rods in the medial of the three vertebral bodies), there maybe advantages to providing various forms of mechanical interactionbetween the axial rods for two adjacent intervertebral spaces.

The previously unfulfilled needs to provide therapy to two or moreadjacent motion segments accessed through a trans-sacral approach areaddressed by the present disclosure. Inventive concepts are illustratedin a series of examples, some examples showing more than one inventiveconcept. Individual inventive concepts can be implemented withoutimplementing all details provided in a particular example. It is notnecessary to provide examples of every possible combination of theinventive concepts provided below as one of skill in the art willrecognize that inventive concepts illustrated in various examples can becombined together in order to address a specific application.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1 is a side view of a human spine;

FIGS. 2A, 2B, and 2C show a trans-sacral approach and the formation of achannel for receipt of therapeutic devices;

FIG. 3 shows one embodiment of a distal distraction rod;

FIG. 4 shows an installed assembly with a distal distraction rod and aproximal distraction rod;

FIG. 5 shows one embodiment of a proximal distraction rod;

FIG. 6A shows an assembly of a distal distraction rod and a proximaldistraction rod along with optional plugs;

FIG. 6B shows a quarter-round cut-away perspective drawing of anassembly with a thrust bearing 680;

FIG. 7 shows another plug that joins the distal and proximal rods;

FIG. 8 shows yet another plug embodiment that uses a separate plugwithin the proximal rod that does not connect the proximal rod to thedistal rod;

FIG. 9 presents a flow chart for the process of imposing twodistractions on adjacent intervertebral spaces;

FIG. 10 illustrates the use of the present disclosure for a situationcalling for the consecutive therapy of three adjacent motion segments;

FIG. 11 shows a two-level therapy provided by the sequentialinstallation of two axially implantable rods;

FIG. 12 shows a two-level therapy provided by the sequentialinstallation of two axially implantable rods but does so withoutanchoring to the vertebral body beyond the most cephalad intervertebraldisc space to receive the therapy;

FIG. 13 shows the use of the present disclosure to provide dynamicstabilization to two adjacent motion segments; and

FIG. 14 shows a preferred driver for insertion or removal of plugs asthis driver has a retention rod to engage the plug to the distal tip ofthe driver.

DETAILED DESCRIPTION

As the present disclosure is an extension of earlier work by TranS1 Inc.that has been well documented by a series of patent applications thathave been incorporated by reference, this discussion will focus on theaspects of the disclosure that are new and the particularly relevantaspects of the previously described work that are useful for explainingthe new material. The general method for establishing a channel via atrans-sacral approach has been well documented and is generallyapplicable to the present disclosure as the inventive axial spinalstabilization rods and assemblies disclosed in this application aredeployed via substantially the same trans-sacral access, usingpreparations, methods, and surgical tools and instrumentation setsdescribed previously in co-pending and commonly assigned U.S. patentapplication Ser. Nos. 10/972,065, 10/971,779, 10/971,781, 10/971,731,10/972,077, 10/971,765, 10/971,775, 10/972,299, 10/971,780, all of whichwere filed on Oct. 22, 2004, and in co-pending and commonly assignedUnited States Provisional Patent Application “Method & Apparatus forAccess & Deployment of Spinal Stabilization Devices Through Tissue,”filed Aug. 9, 2005, as U.S. Provisional Pat. App. No. 60/706,704, all ofwhich are incorporated herein by reference in their entirety.

As noted previously, all steps in this surgical technique use activereal time imaging, and preferably by radio-imaging means such as biplanefluoroscopy, and generally the inventive axial rods of the presentdisclosure are cannulated for delivery of device assemblies by means ofdeployment over an extended guide pin, for an atraumatic introductionthrough soft tissue through an exchange cannula that has been advancedinto its proper target location. Those interested in the details ofthese preparatory steps can review co-pending and commonly assigned U.S.Provisional Application No. 60/601,842 filed Aug. 14, 2004 for Method &Apparatus for Multi-Level Stabilization of the Spine including pages21-27 which are incorporated herein by reference. Another discussion ofrelevant preparations can be found in commonly assigned U.S. Pat. No.6,921,403 “Method and Apparatus for Spinal Distraction and Fusion”issued on Jul. 26, 2005 the relevant portions are incorporated herein byreference.

FIG. 3 shows one embodiment of an exemplary distal distraction rod 300in accordance with the present disclosure. (As will be discussed ingreater detail below, the disclosure is not limited to situations wherethe distal device is anchored in two different vertebral bodies or evenif so anchored, is used to impose a distraction.) FIG. 3A shows theexterior, FIG. 3B provides a cross section, and FIGS. 3C and 3D providesolid surface views in perspective with a longitudinal section removedto expose the interior cavity 304. The operation of the distaldistraction rod is best explained in connection with spinal componentsas shown in FIG. 4. FIG. 4 shows three adjacent vertebral bodies calledherein distal vertebral body 404, medial vertebral body 408, andproximal vertebral body 412. The three vertebral bodies define twoadjacent motion segments, comprising intervertebral disc spaces, thedistal intervertebral disc space 416 and the proximal intervertebraldisc space 420. Note that the proximal vertebral body is drawn without acomplete outline as the three vertebral bodies are not meant to belimited to specific vertebral bodies. Thus, the proximal vertebral body412 is not necessarily the sacrum 116 in FIG. 2C as the axialtrans-sacral channel 212 may have been extended sufficiently into thespine so that the most distal vertebral body 404 is L3 or higher.

The distal axial rod 300 is comprised of a distal threaded section 308,a proximal threaded section 312 and in this preferred embodiment, awaist section 316. The use of dissimilar thread pitches allows for thecontrolled distraction of two vertebral bodies (FIG. 4 elements 404 and408) when the distal threaded section 308 is engaged with a distalvertebral body 404 and the proximal threaded section 312 is engaged withthe medial vertebral body 408.

The use of dissimilar thread pitches to distract vertebral bodies withina single motion segment is described in co-pending and commonly assignedU.S. patent application Ser. No. 10/309,416 filed on Dec. 3, 2002, nowU.S. Pat. No. 6,921,403 which is incorporated herein in its entirety byreference into this disclosure. The use of dissimilar thread pitches canbe used in the distal axial rod 300 which is advanced into the vertebralbodies (404 and 408) by rotating the trailing end of the distal axialrod in the same direction as the “handedness of the screw”. The threadon the proximal threaded section 312 and the thread on the distalthreaded section 308 both extend counterclockwise (or both clockwise)around the elongate body comprising the device, and preferably thedistal and proximal threads are self-tapping. A distal axial rod 300having a thread pitch in its distal threaded section 308 that is finerrelative to the thread pitch in the threaded proximal section 312 causesdistraction of the intervertebral disc space 416 between the two engagedvertebral bodies 404 and 408 as each turn of the distal axial rod 300 inthe proper direction with respect to handedness of the threads will movethe distal axial rod 300 relative to the distal vertebral body 404 afirst amount but the distal axial rod 300 will move relative to the moreproximal vertebral body 408 (medial vertebral body) a larger amount. Theratio of the first amount to the larger amount will be proportional tothe ratio of the pitch of the distal threaded section to the pitch ofthe proximal threaded section. One of skill in the art can appreciatethat in order to effect a more significant distraction; one would selectmore significantly dissimilar thread pitches than the combination shownin FIG. 3. As rotation in one direction causes distraction, rotation inthe opposite direction causes compression.

The preferred embodiments of distal axial rod 300 include a chip breakersection 320 to facilitate screwing the distal end of the distaldistraction rod into the distal vertebral body. The leading edge 324 ofthe thread for the proximal threaded section grows from the minordiameter to the major diameter of the threaded section. The cavity 304of the distal distraction rod 300 includes several apertures 332 thatextend radially outward at the waist 316. These apertures can be used todeliver material as part of providing therapy to the motion segment,including bone paste or other materials to promote fusion. The distalend 328 of cavity 304 is not generally used as an aperture for deliveryof therapeutic material as this distal end 328 would be positioned in avertebral body rather than in an intervertebral space. The opening atthe distal end 328 is useful when deploying the distal distraction rodover a guide wire.

Note that if the major diameter of the threads in the distal threadedsection 308 is less than the minor diameter of the threads in theproximal threaded section 312 then the channel prepared for theinsertion of the distal distraction device can be of a smaller crosssection as the channel enters the distal vertebral body than the crosssection of the channel through the medial vertebral body such that thedistal section of the distal distraction device can pass through themedial vertebral body without having to be screwed through. While thismay appear attractive to pass the distal threaded section through themedial vertebral body in that the bone around the channel is not marredor otherwise weakened, this is not a preferred practice. A preferredpractice is to use a sequence of decreasing major diameters, but notnecessarily to the extent that the more distal thread sets can be passedthrough without rotating the rod.

It has been found that the engagement of a first set of threads with amajor diameter that is small relative to the size of the bore throughthe vertebral body but yet engages the bore through a vertebral bodydoes not adversely impact the ability to thread a subsequent set ofthreads that have a larger major diameter into that same vertebral body.The advantage of not stepping down the major diameters to such asignificant degree that one set of threads can pass through another moreproximal bore in a vertebral body without screwing the threaded rodthrough the bore is that the range of thread sizes is less extensive. Anextensive range of thread sizes when using three or more sets of threadsforces a choice between using rods that have distal sections that arerelatively thin or ending with rods that are very thick.

Rod driver engaging zone 336 can be made in one of severalconfigurations known to those of skill in the art to allow a driver toimpart rotation to the rod. For example, the proximal end of the distaldistraction rod 300 can be fitted with a female hex head suitable fordriving with a driver having a corresponding male hex head. A suitabledriver is described in priority document No. 60/601,842 filed Aug. 14,2004 for Method & Apparatus for Multi-Level Stabilization of the Spineand the relevant portions of that document including FIG. 28 areincorporated herein by reference.

In a preferred embodiment the distal distraction rod 300 may beconfigured to have a set of female extraction threads 348 (for exampleleft-handed metric thread pattern M7) within the proximal end of thecavity 304. The thread matches the thread on an extraction driver tool.

It is likely that the process to extract a previously inserted rod wouldstart with using the driver to rotate the rod in the counterclockwisedirection to start the extraction of the previously inserted rod. (Insome cases the extraction could be performed without the initialinvolvement of the driver to start the disengagement.) Once the distaldistraction rod is backed out slightly with an axial rod driver toolthat engages with the rod driver engaging zone 336, the extraction toolcan be used to engage the female extraction threads 348 and pull thedistraction rod out the rest of the way, if there is a need for suchextraction, e.g., in the event of revision or implant selectionresizing. The use of left-handed threads is preferred as this allows theextraction of the engaged right-handed threads of the rod to bedisengaged by rotating the extraction tool in the normalcounterclockwise direction. By using left-handed threads,counterclockwise rotation will cause the left-handed threaded section ofthe distal tip of the extraction tool to engage with correspondingthreads on the rod to be extracted. After the left-handed threads arefully engaged, further rotation of the extraction tool in thecounterclockwise direction will disengage the right-handed threads onthe rod from the vertebral body. Once the rod is disengaged from thevertebral body, it can be pulled out with the tip of the extraction toolas it will be engaged with the left-handed threads of that tool.

In a particularly preferred embodiment, the left-handed threads are cutinto the polygonal walls of the rod driver engagement zone (perhaps bestseen in FIG. 6B described below). The use of extraction tools withleft-handed threads to remove a rod is not limited to distal distractionrods, but can be used for any installed device that is engaged withright-handed threads including plugs (described below). One of skill inthe art will appreciate that if the devices are engaged into vertebralbodies or into other devices using left-handed threads for theengagement, then an extraction tool would have right-handed threads sothat clockwise rotation of the extraction tool would disengage such adevice.

The use of female extraction threads has been discussed in connectionwith a distal distraction rod 300 as an example of this aspect of thepresent disclosure, but the concept is applicable to other axiallyinserted devices.

FIG. 5 shows an exemplary proximal distraction rod 500 for use with thepresent disclosure. More specifically, FIG. 5A shows the exterior of theproximal distraction rod 500; FIG. 5B shows a cross section along thelength of the proximal distraction rod 500; and FIG. 5C provides a solidsurface perspective view of the proximal distraction rod 500 with alongitudinal segment removed. Proximal distraction rod 500 is comprisedof a threaded section 504 and an engagement section 508. While not shownin this embodiment, one of skill in the art could reduce the length ofthe threaded section so that the threaded section ends before thebeginning of the engagement section, e.g., by including a waist as seenin the exemplary distal distraction rod 300.

The threaded section 504 has a tapered section 512 and a straightsection of thread 516 as this configuration facilitates threading theleading edge of the threaded section into the proximal vertebral body412.

The engagement section is not threaded but has a tapered leading edge520. As the proximal distraction rod 500 is advanced in the channel, thetapered leading edge 520 of the engagement section 508 engages with theproximal end of the distal distraction rod 300. The engagement section508 proceeds into the distal distraction rod cavity 304 until theshoulder 524 of the proximal distraction rod presses against thetrailing edge of the distal distraction rod 300.

In a preferred embodiment, the distal engagement section 508 of theproximal distraction rod essentially fills the corresponding portion 346(“engagement zone”) of the bore in the distal distraction rod 300. Inone embodiment the specification for the bore size for the portion toreceive the cylindrical shank is 0.250 inches (+0.005 inches, −0.000inches) and the specification for the dimension of the cylindrical shankis 0.2495 inches (+0.000 inches, +0.005 inches). The angle used for thetapered leading edge 520 portion of the distal engagement section 508 isrepeated in the corresponding section of the bore 304.

This close fit of the leading portion of the proximal distraction rod500 with the trailing portion of the bore 304 in the distal distractionrod serves to maintain the axial alignment of the two rods to oneanother while retaining the ability for the proximal distraction rod 500to rotate relative to the distal distraction rod 300 without imparting arotation to the distal distraction rod and thus altering the previouslyimposed distal distraction. The rotation of the proximal distraction rod500 with threads engaged in the proximal vertebral body 412 advances theproximal distraction rod 500 which pushes on the distal distraction rod300 to push the distraction rod and the engaged distal 404 and medial408 vertebral bodies away relative to the proximal vertebral body 412 toimpose a desired amount of distraction of the proximal intervertebraldisc space 420.

Note that with the method as described, the amount of distractionimposed on the proximal intervertebral disc space 420 is independent ofthe amount of distraction imposed on the distal intervertebral discspace 416. Note further, that the pitch of the thread on the threadedportion 504 of the proximal distraction rod is not relevant to theamount of distraction that can be imposed (beyond changing the amount ofdistraction per turn of the distraction rod). In fact, the handedness ofthe thread for the threaded portion 504 of the proximal distraction rodcan be chosen independent of the handedness of the thread used for theproximal distraction rod so that distraction is imparted by rotating thedistal distraction rod in a first direction and distraction is imposedby rotating the proximal distraction rod in the opposite direction.

Optionally, the cross section of the proximal distraction rod 500 can beselected to be sufficiently larger than the major diameter of theproximal threaded section 312 of distal distraction rod 300 to allow thecross section of the channel formed in proximal vertebral body 412 to besized so that the distal distraction rod 300 can be passed through theproximal vertebral body 412 without being screwed through it orotherwise marring the bone surface exposed by the channel.

Proximal distraction rod 500 has a set of apertures 532 connected to thecavity 528 of the proximal distraction rod 500. These apertures can beused to distribute therapeutic material as part of the procedure ofmotion segment fusion. In a preferred embodiment, there are fourapertures spaced 90 degrees apart.

Rod driver engaging section 536 can be made in one of severalconfigurations known to those of skill in the art to allow a driver toimpart rotation to the rod. For example, the proximal end of theproximal distraction rod 500 can be fitted with a female hex headsuitable for driving with a driver having a corresponding male hex head.A suitable driver is described in priority document No. 60/601,842 filedAug. 14, 2004 for Method & Apparatus for Multi-Level Stabilization ofthe Spine and the relevant portions of that document including FIG. 28are incorporated herein by reference.

A set of female extraction threads 548 (preferably left-handed metricthreads) at the proximal end of the bore of the proximal distraction rod500 can be used for the extraction of the proximal distraction rod asdiscussed in connection with female extraction threads 348.

Female threaded section 540 for use in securing a bore plug in thecavity 528 will be discussed in greater detail below.

Cavity Plugs.

The purpose of the axial rod plug is to preclude leakage or migration ofthe osteogenic, osteoconductive, or osteoinductive gel or paste which isinserted by means of an augmentation media (e.g., bone paste; PNmaterial) inserter through apertures from the cavity of the distal orproximate distraction rods into the intervertebral spaces as part of theprocess of promoting fusion or for other therapeutic purposes. Often thematerial inserted in this way is intended to fill available volume notoccupied, e.g., by previously introduced autologous bone graft material,in its entirety. In a preferred aspect of the present disclosure, theplug is fabricated from the same titanium alloy as the axial rod,although it may be formed from other suitable (e.g., biocompatible;polymeric) materials.

A) Interlocking Cavity Plugs.

FIG. 6A presents a representation of a distal distraction rod 300 with aproximal distraction rod 300 shown inserted into the proximal end of thecavity of the distal distraction rod. As noted above, the cavity 304 ofthe distal distraction rod is connected to apertures 332 and it may bedesirable to plug the cavity 304 to prevent or limit the ingress ofmaterial into the cavity 304 including the post-treatment ingress oftherapeutic material delivered through these apertures. A distal rodplug 604 is shown (not to scale) with a male threaded section 608 thatcorresponds to female threaded section 340. The distal rod plug 604 canbe driven by a hex driver that is appropriately sized to drive a femalehex fitting 612 in the trailing edge of the cavity in the distal rodplug 604. A suitable driver is described in priority document No.60/601,842 filed Aug. 14, 2004 for Method & Apparatus for Multi-LevelStabilization of the Spine and the relevant portions of that documentincluding FIG. 29 are incorporated herein by reference. The distal rodplug 604 when installed in the distal distraction rod 300 is seateddistal to the tapered section 344 of the cavity so that the installeddistal rod plug does not interfere with the insertion of the proximaldistraction rod 500 into the proximal end of the distal distraction rod300.

The cavity in the distal rod plug 604 has a female threaded section 616which will be described in connection with the proximal distraction rodplug 650.

Proximal rod plug 650 has male threaded section 654 which is adapted toengage female threaded section 616 of distal rod plug 604 to bindtogether the assembly including distal distraction rod 300 with distalrod plug 604 along with proximal distraction rod 500 to provide onerigid assembly. Note that in the preferred embodiment, the proximal rodplug 650 does not engage via male threads with female threaded section540 in the proximal end of the cavity of the proximal distraction rod500. By not engaging with a second set of threads in a different axialrod that is free to rotate with respect to the distal rod, there is norisk of cross-threading or working out an alignment method to align thefemale threaded sections to one another.

Proximal rod plug 650 in turn has an axial cavity with a female threadedsection 658. This threaded cavity is used in connection with a preferredplug driver described below that uses a retention rod to engage with theplug so that it remains engaged with the distal tip of the driver untilthe driver is disengaged from it.

The proximal rod plug 650 can be driven by a hex driver that isappropriately sized to drive a female hex fitting 662 in the trailingedge of the cavity in the proximal rod plug 650. A suitable driver isdescribed in priority document No. 60/601,842 filed Aug. 14, 2004 forMethod & Apparatus for Multi-Level Stabilization of the Spine and therelevant portions of that document including FIG. 29 are incorporatedherein by reference.

FIG. 6B shows a perspective view with a quarter section removed of anassembled combination of a proximal rod 500, a distal rod 300, a distalrod plug 604 and a proximal rod plug 650 that engages with the proximalend of the distal rod plug 604. Note that the components have been sizedto allow for the use of a thrust bearing 680 which serves to facilitate(e.g., lubricate) the rotation of the proximal rod 500 against thedistal rod 300 so that the proximal rod can advance and rotate withoutimparting rotation to the previously installed distal rod 300.

The thrust bearing can be as shown here as a washer shaped structure. Asthe thrust bearing will be placed inside a human body, it should be madeof a biocompatible material and tolerant of the forces it may see inuse. As the thrust bearing is meant to facilitate the rotation of themore proximal rod 500 relative to the more distal rod 300 while under anaxial load, the coefficient of sliding friction between the thrustbearing and the rod moving relative to the thrust bearing should be lessthan the coefficient of sliding friction between two rods as shown inFIG. 6A.

An example of a material considered appropriate for the thrust bearingis ultra high molecular weight polyethylene (UHMWPE) another viable butless preferred material is polyether ether ketone known as PEEK.

The perspective view shown in FIG. 6B includes the female extractionthreads 348 for the distal distraction rod 300 and a better view of thefemale extraction threads 548 for the proximal distraction rod 500. Notein the preferred embodiment the female extraction threads such as 548are cut into the most proximal section of a polygonal rod driverengaging section 536.

B) Single Trans-Rod Plug.

An alternative embodiment of the present disclosure uses a single plug,but uses one that locks together the distal distraction rod 300 with theproximal distraction rod 500. Plug 700 has a male threaded section 704that is adapted to engage with female threaded section 340 when anappropriate driver (not shown in FIG. 7) presses upon and rotates theplug through interaction at female socket 708. The cavity of plug 700has female threaded section 712. When fully inserted, the tip 716 ofplug 700 is beyond the apertures 332 so as to block the ingress ofmaterial back through those apertures or through apertures 532 of theproximal distraction rod.

In certain situations the pair of plugs 604 and 650 may be preferable toplug 700 as the insertion of distal rod plug 604 seals off the apertures332 before the proximal distraction.

C) Two Independent Plugs.

FIG. 8 illustrates a third embodiment in which the ingress of materialinto the proximal distraction rod 500 is limited by the use of a plug804 that is engaged with the proximal distraction rod 500 rather thanthe distal distraction rod 300 or a plug within distal distraction rod300. More specifically, FIG. 8A shows a partial cross section and FIG.8B shows a side-elevational perspective view of a proximal distractionrod 500 inserted into the proximal end of a distal distraction rod 300.The distal distraction rod 300 is sealed internally with a plug 804 anda proximal rod plug 850 is located in the proximal distraction rod 500.In each case, a male threaded section 808 and 858 engage correspondingfemale threaded sections 340 and 540 (best seen in FIG. 6) to secure theplug (804 or 850) into the cavity of the distraction rod. The plugs aredriven by appropriate drivers that engage the engagement sections 812and 862 (preferably female hex fittings). The tips 816 and 866 of theplugs are placed so that material cannot travel from the apertures backinto the longitudinal cavities of the distraction rods.

There are advantages in reducing the number of different unique parts inconfiguring the distraction rods such that a single plug could be usedin either the proximal or distal distraction rods and in using the sameplug for a range of different length distraction rods (or in the case ofthe distal distraction rod for distraction rods with different pairingsof proximal and distal thread pitches).

Flow Chart.

After the introduction of the various components and concepts set forthabove, it may be helpful to review the presented material through thecontext of a flowchart. FIG. 9 presents a flow chart for the process ofimposing two distractions on adjacent intervertebral spaces.

Step 905 calls for preparation of the channel to allow for the insertionof the axial distraction rods.

Step 910 calls for engaging the threads from the distal threaded section308 with the distal vertebral body 404 and the proximal threaded section312 with the medial vertebral body 408.

Step 915 calls for rotating the engaged distal distraction rod 300 byapplying force to the rod driver engaging zone 336 to selectively imposea specific amount of distraction to the distal intervertebral spacethrough the action of the differences in thread pitch between the distalthreaded section 308 and the proximal threaded section 312. Note thatthe direction of rotation to impose a distraction is a function of thehandedness of the threads and whether the finer pitch thread is on theproximal or distal set of threads.

Step 920 calls for applying the desired therapy to the distracted distalintervertebral space 416. This therapy may include providing materialsto the distal intervertebral space 416 through the apertures 332.

Step 925 calls for the optional addition of a distal distraction rodplug such as distal distraction plug 604. This is optional as somemedical providers may opt to not seal the cavity at all and some mayrely on a plug applied after the proximal distraction that will sealboth axial distraction rod cavities.

Step 930 calls for threading the proximal distraction rod 500 into theproximal vertebral body 412.

Step 935 calls for rotating the proximal distraction rod 500 until itpushes against the engaged distal distraction rod to impose a specificamount of distraction upon the proximal intervertebral disc space 420.Preferably the distal end of the proximal distraction rod 500 engageswith the proximal end of the cavity in the distal distraction rod 300 sothat the application of force by the proximal distraction rod 500against the distal distraction rod 300 occurs without disturbing theaxial alignment of the two distraction rods. Note that the amount ofdistraction imposed on the proximal intervertebral disc space is notdependent on the amount of distraction imposed on the distalintervertebral space.

Step 940 calls for the application of the therapy to the proximalintervertebral disc space 420 which may include the insertion ofmaterial into the proximal intervertebral space through the apertures532.

After the application of the therapy to the proximal intervertebralspace there are several options for the application of a plug. Oneoption not explicitly set forth on the flow chart is to not insert anyplug at all in the proximal distraction rod. Steps 945, 950, and 955provide alternatives that can be selected to insert three differenttypes of plugs into the proximal distraction rod.

Step 945 calls for the addition of a proximal distraction rod plug suchas shown in FIG. 8 as element 850. Such a plug seals the cavity of theproximal distraction rod but does not serve to join the proximaldistraction rod 500 with the distal distraction rod 300 (either directlyor indirectly through a distal plug).

Step 950 calls for the insertion and engagement of a proximaldistraction plug with the distal distraction rod 300. This wasillustrated by element 700 in FIG. 7 which engages with the femalethreaded section 340 in the distal distraction rod 300.

Step 955 calls for the insertion and engagement of a proximaldistraction plug with the distal distraction rod plug such as is shownin FIG. 6 as proximal distraction rod plug 650 engages with previouslyinserted distal distraction rod plug 604 to provide added stability byjoining the proximal distraction rod 500 to the distal distraction rod300.

Note that when discussing the plugs referenced in Steps 950 and 955 andwhen focused on the structural contributions of such plugs, it isperhaps more appropriate to refer to the device as an inter-rodconnector as one of ordinary skill in the art can appreciate that aninter-rod connector provides a function of connecting the two rodstogether whether or not sealing is desired or even provided by theinter-rod connector. Thus, a connector between two axial distractionrods in keeping with the teachings of the present application areintended to be within the scope of the claims whether or not such aconnector serves a purpose of acting as a “plug” to limit the ingress ofmaterial into the cavity through an aperture as some distraction rodsmay not have apertures and some therapies may not call for the insertionof therapeutic material through the apertures.

Distraction of Three or More Adjacent Intervertebral Spaces.

FIG. 10 illustrates the use of the present disclosure for a situationcalling for the consecutive therapy of three adjacent motion segmentsThe three rod assembly is shown in outline but includes indications ofaspects of the interior cavities including threads, engaging sectionsfor rod drivers and the engagement zones and engagement sections such asengagement zone 346 of the distal distraction rod 300 and thecorresponding engagement section 1058 of the next distraction rod(described below).

In this case the four vertebral bodies illustrated in FIG. 10 are thedistal vertebral body 404 (L3), distal-medial vertebral body 1004 (L4),proximal-medial vertebral body 1008 (L5), and proximal vertebral body412 (sacrum). The three intervertebral disc spaces between the fourvertebral bodies are the distal intervertebral disc space 416, themedial intervertebral disc space 1012, and the proximal intervertebraldisc space 420. This embodiment has three axial distraction rods, thedistal distraction rod 300, a medial distraction rod 1050, and aproximal distraction rod 550.

The insertion of the distal distraction rod 300 to distract the distalintervertebral disc space 416 through the use of two sets of threads ofdifferent pitches operates as described above. Naturally, the lengths ofthe distal threaded section 308, proximal threaded section 312, andwaist 316 will be adjusted to be appropriate for whatever motion segmentis targeted whether it is L3/L4 instead of L4/L5.

As the threaded section 1054 of the medial distraction rod 1050 engagesthe proximal-medial vertebral body 1008, continued rotation appliedthrough an engagement between a driver and a corresponding rod driverengagement zone in the proximal cavity of the medial distraction rodcauses the medial distraction rod 1050 to advance and push against theproximal end of the distal distraction rod 300. This advancement andpushing causes an enlargement of the intervertebral space, in this casethe medial intervertebral disc space 1012. After the distraction, theapertures 1062 are positioned in the intervertebral space so thattherapeutic material can be delivered to this space.

In a preferred embodiment the distal end of the medial distraction rod1050 will have an engagement section 1058 that fits with closetolerances within the engagement zone 346 proximal end of cavity 304within the distal distraction rod 300.

The medial distraction rod 1050 is characterized by having an engagementsection 1058 to engage with a correspondingly shaped portion of a cavityof a more distal rod and an engagement zone 1066 in a portion of thecavity at the proximal end of the medial distraction rod 1050 that isadapted for receiving the engagement section 566 of a more proximaldistraction rod. In short, the medial distraction rod 1050 is adapted topush against a more distal distraction rod and to be pushed by a moreproximal distraction rod. In keeping with the description set forth inconnection with the distractions rods shown in FIGS. 3 and 5, the medialdistraction rod 1050 would also include a rod driver engagement zone inthe proximal end of the cavity for use by a corresponding driver and mayinclude female threads 1072 for use by an extraction tool.

A proximal rod 550 is subsequently threaded into the proximal vertebralbody 412. The proximal rod 550 in this embodiment has apertures 562 influid communication with the internal cavity of the proximal distractionrod 550. The proximal distraction rod 550 would also include a roddriver engagement zone in the proximal end of the cavity for use by acorresponding driver and may include female threads 572 for use by anextraction tool.

While the two examples given above in connection with FIGS. 4 and 10show a series of fusion therapies applied to two or three adjacentmotion segments, the disclosure is not limited to fusion therapy.Another class of therapies works to provide for some degree ofpost-operative mobility in the treated motion segment. The specifics ofthe therapeutic aspects of such devices have been described in detail inthe earlier applications referenced above. As the focus of thisapplication is on the ability to provide therapy to two or more adjacentmotion segments, the details of the operation of these axial spinalmobility preservation devices (also termed dynamic stabilization ormotion management, or “MM” devices) are not repeated here.

FIG. 11 shows a two-level therapy provided by the sequentialinstallation of two axially implantable rods. Distal rod 1104 iscomprised of a distal threaded section 1108, a therapeutic section 1112,and a proximal threaded section 1116. The distal threaded section 1108and the proximal threaded section 1116 have the same thread pitch inthis example and thus the insertion of the distal rod 1104 would notimpose a distraction on the distal intervertebral disc space 416 as thedistal rod 1104 engaged with distal vertebral body 404 and medialvertebral body 408.

Therapeutic section 1112 is shown here in outline after material hasbeen inserted into the intervertebral space and retained by devicemembrane 1120. In order to make device membrane 1120 distinguishable inthis drawing from the components in the motion segment, a small spacehas been left in the drawing between the device membrane and the othercomponents in the motion segment. This is for purpose of illustrationonly as the expanded device membrane 1120 would conform to the shape ofthe intervertebral space. Subsequent illustrations showing analogousdevice membranes will likewise exaggerate the spacing between the devicemembrane and other components for the same reason.

The proximal rod 1150 shown in FIG. 11 has a threaded section 1154 andapertures 1158. The engagement between the distal and proximal rods isnot visible in this drawing but can be done in the same manner asdescribed above in connection with FIG. 4. Likewise the options forplugs are as described above. FIG. 11 does illustrate the point that thehandedness of the threaded section 1154 does not need to be the same asthe handedness of the threads used in the distal rod 1104 in order toachieve a distraction as the more proximal rod will push in the cephaladdirection on the implanted more distal rod to increase the axialdistance between the vertebral body engaged with the more proximal rodand the vertebral body or bodies engaged with the more distal rod.

FIG. 12 has a distal rod 1204 that has only one threaded section 1208.Thus the distal rod is engaged with the medial vertebral body 408 butnot with the distal vertebral body 404. A therapeutic section 1212 ofthe distal rod 1204 extends into the distal intervertebral disc space416. After insertion of material into expandable device membrane 1220,the device expands to a conforming fit within the intervertebral discspace 416. While this distal rod 1204 did not impose a distractionthrough the use of two threaded sections of dissimilar thread pitch, oneof skill in the art will recognize that some distraction could beimposed hydraulically by expanding a membrane within intervertebral discspace 416. Proximal rod 1150 of FIG. 12 can be configured internally andoperate as described in connection with FIG. 11.

FIG. 13 illustrates yet another use of the present disclosure to providetherapy to two adjacent spinal motion segments. More specifically, FIG.13 shows a distal rod 1204 with the various elements described inconnection with FIG. 12. FIG. 13 differs from FIG. 12 in that theproximal rod 1350 is not used to fuse the motion segment of proximalvertebral body 412, medial vertebral body 408, and proximalintervertebral disc space 420 as was done in FIGS. 11 and 12. InsteadFIG. 13 shows two adjacent motion segments receiving motion managementtherapy to provide for post-operative mobility in both of the treatedmotion segments. Proximal rod 1350 would be advanced axially towardsdistal rod 204 by engaging the threaded section 1354 with the proximalvertebral body 412 through use of an appropriate rod driver and a roddriver engagement section in the proximal end of a longitudinal cavityin proximal rod 1350. After proximal rod 1350 engages with distal rod1204 any subsequent rotation of proximal rod 1350 to advance the rod inthe cephalad axial direction would cause the distal rod 1204 and theengaged medial vertebral body 408 to move axially away from proximalvertebral body 412. After achieving the desired level of distraction (ifany) of the proximal intervertebral space, material could be providedinto the proximal end of the cavity in proximal rod 1350 and outapertures in the therapeutic section 1362 of proximal rod 1350 to expanddevice membrane 1370.

While the preceding examples used motion management therapies that usedmembranes to contain the prosthetic nucleus material inserted into theintervertebral spaces through apertures in the rods, the disclosure isnot limited to these particular types of motion management devices.Other motion management therapies as described in the various co-pendingapplications or issued patents and their priority documents can becoupled with the teachings of the present disclosure to provide therapyto two or more adjacent spinal motion segments. For example, motionmanagement therapies that inject prosthetic nucleus material into theintervertebral space without a membrane are specifically included in theintended range of uses for the present disclosure.

TABLE A Multi-level L3-L4 L4-L5 L5-S1 Configuration L3 Disc L4 Disc L5Disc S1 Figure #s 3-Level Anchor MM + PN Anchor MM + PN Anchor MM + PNAnchor Anchor MM + PN Anchor MM + PN Anchor Fusion Anchor Anchor MM + PNAnchor Fusion Anchor Fusion Anchor Provisional FIG. 14a&b Anchor FusionAnchor Fusion Anchor Fusion Anchor 10 PN + DD Anchor MM + PN Anchor MM +PN Anchor PN Anchor MM + PN Anchor MM + PN Anchor PN + DD Anchor MM + PNAnchor Fusion Anchor PN Anchor MM + PN Anchor Fusion Anchor ProvisionalFIG. 10 PN + DD Anchor Fusion Anchor Fusion Anchor Provisional FIG. 13PN Anchor Fusion Anchor Fusion Anchor Provisional FIG. 15 2-Level AnchorMM + PN Anchor MM + PN Anchor Anchor MM + PN Anchor Fusion Anchor 11Anchor Fusion Anchor Fusion Anchor 4 could apply here but 4 is notlimited to these vertebrae. PN + DD Anchor MM + PN Anchor PN Anchor MM +PN Anchor 13 PN + DD Anchor Fusion Anchor Provisional FIG. 12 PN AnchorFusion Anchor 12

Table A is provided to highlight the various non-exhaustive examples ofthe range of applications of the teachings of the present disclosure.The table references the examples in this application and additionalexamples in Provisional Application No. 60/601,842 filed Aug. 14, 2004for Method & Apparatus for Multi-Level Stabilization of the Spine whichhas been incorporated by reference. The various drawings referenced inthe table and the text associated with those drawings in the provisionalare all incorporated herein by reference. In this table anchor refers toa threaded portion of a device or rod that engages a vertebral body; PNrefers to prosthetic nucleus.

Plug Driver with Retention Rod.

FIG. 14 shows a preferred driver for insertion or removal of plugs ofthe various types discussed above. FIG. 14A shows the driver assembly1400. The driver assembly 1400 has a handle 1404 that is adapted forproviding rotation to the driver assembly 1400 and consequently to thedriven plug. The handle 1404 is connected to a driver shaft 1408 to forma driver 1412 with polygonal driver head 1416 as shown in FIG. 14B. In apreferred embodiment, the driver head 1416 is part of a removable tip1420 that is attached to the driver shaft 1408, such as by connectionpin 1424.

Retention rod 1450 is comprised of a threaded distal end 1454 and arotation actuator 1458, in this case a knob. The retention rod 1450 canbe inserted into the driver shaft 1408 and the threaded distal end 1454extended through the driver shaft 1408 and out through the driver head1416 as the retention rod 1450 is longer than the driver 1412. FIG. 14Cshows an enlarged detail of the driver head 1416 with the protrudingthreaded distal end 1454.

The advantage of the driver with retention rod 1450 is that a plug canbe engaged to the driver 1400 by engaging the threaded distal end 1454with a corresponding portion of the proximal end of the plug. This canbe done by holding the proximal end of the plug adjacent to the threadeddistal end 1454 and rotating the rotation actuator 1458 to cause thethreaded distal end to rotate relative to the driver shaft 1408 and theheld plug. Once the plug is engaged with the threaded distal end 1454and the driver head 1416, the distal end of the driver assembly 1400 canbe inserted into the channel along with the engaged plug. After thedistal end of the plug is inserted into the relevant device, the handlecan be rotated to engage threads on the plug with threads in the deviceto engage the plug. After the plug is at least partially engaged withthe device in the channel, the rotation actuator 1458 can be rotated tocause the threaded distal end 1454 of the retention rod 1450 to rotaterelative to the driver head 1416 and the plug. Rotation in the properdirection (based on the handedness of the threads used on the threadeddistal end 1454) will disengage the threaded distal end 1454 from theplug. After the plug is installed in the device, the distal end of thedriver assembly 1400 can be withdrawn from the channel.

Extraction of a plug from a device would start with putting the threadeddistal end 1454 of a retention rod 1450 (which is part of a driverassembly 1400) into the channel and adjacent to the proximal end of theplug to be extracted. Rotation of the rotation actuator in theappropriate direction for the threads used will cause the threadeddistal end 1454 to engage with the installed plug. After the threadeddistal end 1454 is engaged with the plug and the plug is engaged withthe driver head 1416 then rotation of the driver assembly 1400 throughthe use of the handle 1404 will cause the plug to disengage from thedevice. After the plug is disengaged, it can be removed with the driverassembly as the driver assembly is removed from the channel because theplug is threadedly engaged with the retention rod 1450.

Properties of preferred materials for axially implantable devices arediscussed at length in the U.S. Provisional Application No. 60/601,842and the relevant material in that application, including material onpages 38-40 of that application, is incorporated herein by reference.

Alternative Embodiments

Throughout this document, there have been references to both male andfemale hex head fittings. Hex fittings are preferred fittings but one ofordinary skill in the art will recognize that other corresponding maleand female fittings can be used to impart rotation from a driver to adriven rod, plug, or other component. With the exception of a perfectcircle, almost any polygon with regular or irregular sides would workincluding: triangle, square, pentagon, heptagon, octagon, et cetera.Other configurations could work including shapes with curves such ascrescents, ovals, semi-circles, or even an array of two or more circlesthat do not share the same center axis. Nothing in this specification orthe claims that follow should be construed as limiting the scope ofclaim coverage to hexagonal drivers.

The preferred embodiment discussed in detail above uses a set of axialdistraction rods so that the cross sections of the threaded sections ofthe rods get progressively smaller. An alternative is that twoconsecutive sets of threads on two different distraction rods can havethe same thread and cross section. For example, if the proximal threadedsection 312 of the distal distraction rod 300 and the threaded section504 of the proximal distraction rod 500 have the same thread pattern,then keying can be useful to prevent cross threading. More specifically,it is necessary to “time” the threads so that they engage the bone atthe same location, to prevent “cross-threading”. Cross-threading canoccur because these are “self-tapping” rods. The first set of threads ofsufficient size to engage the vertebral body (rather than pass throughit) will essentially tap the bore through the vertebral body. As thenext threaded section reaches the previously tapped vertebral body,cross threading will occur unless the leading edge of the thread entersthe track left by the first set of threads in the same place. In oneaspect of the disclosure, an exchange cannula with a threaded innerdiameter that docks with/attaches to the sacrum may be utilized to avoidcross-threading. Each rod can then be readily threaded through thecannula to initially engage the vertebral body at the same location.While this method can be used with an effort to perform two distractionsof adjacent intervertebral spaces, it is particularly useful whenperforming distractions of three or more adjacent intervertebral spacesas keying eliminates the need for four progressively larger crosssections to be made in the sequence of vertebral bodies.

The preferred embodiment of the engagement section is a cylinder with atapered leading edge (frusta-conical) such as shown by engagementsection 508 in connection with FIG. 5. One of ordinary skill in the artwill recognize that a pure cylinder or a cylinder with a rounded leadingshoulder would be a viable solution although that shape would not tendto self-align to the same extent as the preferred embodiment. The entireengagement section could be frusta-conical (that is without acylindrical component), however, the preferred embodiment calls for arapid taper out to a cylinder to increase the wall thickness of theengagement section to increase the strength of this portion of the rod.Likewise, other shapes that allow for endless rotation of the moreproximal rod against the more distal rod would be viable including aleading edge that resembles a hemisphere.

One of skill in the art can appreciate that provided an appropriatechannel that could be created in more than four sequential vertebralbodies, that a sequence of distraction rods could include a distaldistraction rod, two or more medial distraction rods, and a proximaldistraction rod. Assuming that the various rods were sized appropriatelyfor the anatomy of the sequence of motion segments, this would allow forthe sequential distraction and selective application of therapy to fouror more intervertebral spaces.

One of skill in the art will recognize that some of the alternativeembodiments set forth above are not universally mutually exclusive andthat in some cases alternative embodiments can be created that implementtwo or more of the variations described above.

Those skilled in the art will recognize that the methods and apparatusof the present disclosure have many applications and that the presentdisclosure is not limited to the specific examples given to promoteunderstanding of the present disclosure. Moreover, the scope of thepresent disclosure covers the range of variations, modifications, andsubstitutes for the system components described herein, as would beknown to those of skill in the art.

The legal limitations of the scope of the claimed invention are setforth in the claims that follow and extend to cover their legalequivalents. Those unfamiliar with the legal tests for equivalencyshould consult a person registered to practice before the patentauthority which granted this patent such as the United States Patent andTrademark Office or its counterpart.

1. A method to axially distract a first intervertebral space between afirst vertebral body and a second vertebral body, comprising the steps:implanting a first assembly in the first vertebral body through anaccess channel; implanting a second assembly in the second and morecaudal vertebral body through the access channel; and using a rotatingportion of the second assembly to contact a proximal end of the firstassembly to push on the first assembly without rotating the firstassembly to axially distract the first intervertebral space between thefirst vertebral body engaged with the first assembly and the secondvertebral body engaged with second assembly.
 2. The method of claim 1further comprising a preliminary step of creating the access channel foruse in a trans-sacral approach by boring through an anterior face of thesacrum.
 3. The method of claim 2 wherein the first assembly and thesecond assembly are each adapted for delivery through a single accesschannel created during surgery with an opening at a proximal end of theaccess channel that receives the first assembly and the second assembly.4. The method of claim 1 wherein the portion of the second assembly fitswithin an engagement zone in the proximal end of the first assembly. 5.The method of claim 1 wherein the first assembly comprises a first setof male threads for engagement with a distal vertebral body and a secondset of male threads with the same handedness for engagement with a lessdistal vertebral body.
 6. The method of claim 1 wherein the firstassembly comprises a distal tip of the first assembly for placement inan intervertebral space cephalad to the first assembly and the firstassembly is used to insert material into the intervertebral spacecephalad to the first assembly.
 7. The method of claim 1 furthercomprising inserting of a distal end of a plug through a lumen in thesecond assembly into a cavity in the first assembly before a threadedportion on the exterior of the plug is engaged with a corresponding setof threads.
 8. The method of claim 7 wherein the threaded portion on theexterior of the plug engages a set of female threads in a cavity in thefirst assembly.
 9. The method of claim 7 wherein the threaded portion onthe exterior of the plug engages a set of female threads in a cavity ina first plug previously inserted into the first assembly.
 10. The methodof claim 1 further comprising inserting a plug in the first assembly tolimit movement of material delivered to an intervertebral space throughthe first assembly back into the first assembly.
 11. The method of claim1 wherein the rotating portion of the second assembly contacts a thrustbearing on the proximal end of the first assembly to impart an axialforce through the thrust bearing.
 12. The method of claim 1 whereinimplanting the first assembly in the first vertebral body includesimplanting at least a portion of the first assembly in a more cephaladintervertebral space adjacent to the first vertebral body and a distalend of the first assembly is expanded to fill at least a portion of themore cephalad intervertebral space.
 13. The method of claim 1 whereinimplanting the first assembly in the first vertebral body includesimplanting a threaded fusion rod that engages the first vertebral bodyand an adjacent and more cephalad additional vertebral body such thatthe push from the second assembly axially distracts the firstintervertebral space between first vertebral body engaged with the firstassembly and the second vertebral body engaged with second assemblywithout altering a more cephalad intervertebral space between the firstvertebral body and the additional vertebral body.
 14. The method ofclaim 13 wherein implanting the first assembly causes axial distractionof the intervertebral space between the first vertebral body and theadditional vertebral body through use of dissimilar thread pitches onthe first assembly.
 15. The method of claim 13 further comprising thepreliminary step of creating a trans-sacral axis channel with a caudalto cephalad sequence of vertebral body bores of decreasing crosssectional area for cross sections taken perpendicular to thelongitudinal axis of the axis channel such that the vertebral body boresare sized relative to the major diameters of a first set of threadslocated on a distal portion of the first assembly for engagement withthe additional vertebral body, a second set of threads on a proximalportion of the first assembly for engagement with the first vertebralbody, and a third set of threads on the second assembly for engagementwith the second vertebral body; allowing cephalad axial advancement ofthe first, second, and third set of threads of increasing major threaddiameters through the axis channel without cross-threading.
 16. Themethod of claim 13 further comprising the preliminary step of creating atrans-sacral axis channel with a caudal to cephalad sequence ofvertebral body bores wherein the first vertebral body and the secondvertebral body receive bores with the same cross sectional area forcross sections taken perpendicular to the longitudinal axis of the axischannel wherein the steps of: implanting the first assembly in the firstvertebral body; and implanting the second assembly in the second andmore caudal vertebral body are performed using a common driver and timeddelivery in order to avoid cross threading.
 17. The method of claim 13wherein the additional vertebral body is a L4 vertebra, the firstvertebral body is a L5 vertebra, and the second vertebral body is asacrum.
 18. The method of claim 13 wherein the additional vertebral bodyis a L3 vertebra, the first vertebral body is a L4 vertebra, and thesecond vertebral body is a L5 vertebra.
 19. The method of claim 13wherein the first assembly is used to fuse spinal therapy acts to fusetogether a more cephalad motion segment comprising the additionalvertebral body, the more cephalad intervertebral disc, and the firstvertebral body and material is inserted into the more cephaladintervertebral space to promotes fusion, and the material travelsthrough an interior of the first assembly.
 20. The method of claim 19further comprising fusing a first motion segment comprising the firstvertebral body, the first intervertebral space, and the second vertebralbody after distraction of the first motion segment including insertionof material into the first intervertebral space through the firstassembly wherein the material inserted into the first intervertebralspace promotes fusion.
 21. The method of claim 13 wherein the firstassembly is used in provision of dynamic stabilization of a morecephalad motion segment comprising the additional vertebral body, themore cephalad intervertebral disc, and the first vertebral body andmaterial is inserted into the more cephalad intervertebral space toexpand a flexible portion of the first assembly.
 22. The method of claim21 further comprising fusing a first motion segment comprising the firstvertebral body, the first intervertebral space, and the second vertebralbody after distraction of the first motion segment including insertionof material into the first intervertebral space through the firstassembly wherein the material inserted into the first intervertebralspace promotes fusion.
 23. The method of claim 21 wherein the secondassembly is used to in provisional of dynamic stabilization a firstmotion segment comprising the first vertebral body, the firstintervertebral space, and the second vertebral body and material isinserted into the first intervertebral space to expand a flexibleportion of the second assembly.
 24. A method to selectively providedistraction to two different adjacent motion segments comprising:providing a selected spinal therapy to a first motion segment throughuse of a first assembly engaged with at least a first vertebral body;engaging a second assembly with a second vertebral body immediatelycaudal to the first vertebral body; pushing a proximal end of the firstassembly with a rotating portion of the second assembly to push thefirst assembly axially away from the second vertebral body withoutrotating the first assembly; engaging a third assembly with a thirdvertebral body caudal to the second vertebral body; and pushing aproximal end of the second assembly with a rotating portion of the thirdassembly to push the second assembly axially away from the thirdvertebral body without rotating the second assembly.
 25. The method ofclaim 24 wherein the first vertebral body is the L4 vertebra, the secondvertebral body is the L5 vertebra, and the third vertebral body is thesacrum.
 26. The method of claim 24 wherein a selected distraction isprovided to the first motion segment through use of a threaded rodengaged with both the first vertebral body and an adjacent and morecephalad vertebral body through use of dissimilar thread pitch onthreaded sections of the threaded rod.
 27. The method of claim 26wherein more cephalad vertebral body is the L3 vertebra, the firstvertebral body is the L4 vertebra, the second vertebral body is the L5vertebra, and the third vertebral body is the sacrum.